The present invention relates to an on-site method for treating contaminated drill cuttings before disposal and particularly relates to a method for drying the cuttings to eliminate pollution causing organic material from the cuttings to enable the cuttings to be disposed of into the water at an offshore drilling location.
The utilization of an oil base drilling fluid or "mud" in offshore rotary drilling operations has become more desirable with the increased utilization of directional drilling techniques. With an oil base drilling fluid, the cuttings, besides ordinarily containing moisture, are necessarily coated with an adherent film or layer of oily drilling fluid which may penetrate into the interior of each cutting. The cuttings produced as a result of the rotary drilling operation are carried from the bottom of the bore hole via the flow of drilling fluid. Mechanical devices are employed to separate the drill cuttings from the drilling fluid; however, the mechanical separating devices do not effectively separate the oil from the cuttings. Because of pollution of the environment, whether on water or land, the cuttings cannot be permanently discarded until the pollutants have been removed therefrom.
There are two general techniques which have heretofor been employed for treating the contaminated cuttings in an attempt to make such cuttings ecologically acceptable. The first such technique involves hauling the cuttings from an offshore drilling site to disposal facilities on shore. The added expense involved in hauling the cuttings ashore is substantial, and accordingly, seriously detracts from wide spread commercial application of this technique. Further, this technique may become impractical in bad weather and/or rough seas as for example, which normally occur in the North Sea during the winter months.
The second technique involves treating and disposing of the drill cuttings directly at the offshore drilling site. For obvious reasons, this technique is much preferred to the technique previously described. Numerous systems have been proposed for treating the drill cuttings at offshore drilling sites. However, each of the prior art systems have suffered from one or more deficiencies which have prevented these systems from becoming commercially acceptable.
One of the previously considered systems employed high intensity infrared lamps to thoroughly combust the oil entrained in the cuttings. This approach was considered unsafe due to possible fire hazards resulting from usage of the high intensity lamps.
A second prior system involved washing the cuttings with a detergent to remove the contaminates, separating the washing solution and contaminates from the cuttings and thereafter dumping the clean cuttings into the water. Although the cuttings were cleaned by this system, the system again proved impractical from a commercial standpoint since a new polluting agent was created i.e. the used detergent itself, which had to be properly handled otherwise ecological damage would result from improper disposal.
Another system proposed volatilizing all the entrained hydrocarbons by passing the drill cuttings in heat transfer relation with very hot fluid. Due to problems associated with oxidation at the relatively high heat transfer temperature i.e. approximately 600.degree. F. or higher, and because of the threat of explosion, an inert atmosphere was required in the heat transfer zone.
Yet another system proposed utilizing jets to spray the cuttings with steam to heat the cuttings to a temperature above the boiling point of water, resulting in vaporization of moisture plus distillation of the organic material entrained in the cuttings. Such system is very inefficient as the energy required to convert water into steam is wasted energy. Further, as the steam is employed to both evaporate water entrained in the cuttings plus vaporize the oil, the steam very readily approaches its saturation temperature resulting in unwanted condensation of some of the steam. Further, depending upon the quantity of moisture entrained in the cuttings, there may be insufficient supply of steam available to vaporize the organic material after the moisture has been evaporated.
It has recently been recognized that not all hydrocarbons are deleterious to the environment. In particular, it has been found that the light, more volatile hydrocarbons are generally more harmful to marine life and vegetation than are the heavier hydrocarbons. Accordingly, the elimination through combustion or otherwise of all hydrocarbons from the drill cuttings to permit disposal thereof directly into the water surrounding an offshore drilling site is not necessary. Only the light hydrocarbons must be eliminated to permit the cuttings to be disposed of into the water.